504 - Gateway timeout not showing custom error page

[xSlok]

Hello all,
I'm trying to use this gem on my rails project and it works nicely for other errors, but when a gateway timeout occurs, instead of showing my custom error page, the default "We're sorry, but something went wrong" page is shown.
my application.rb setting is like this:

  config.exception_handler = {
      dev: nil,
      exceptions: {
        :all => {
          layout: 'exception', # define layout
        },
        504 => {
          layout: 'exception',
        },
      },
    }

Am I missing something?
Thanks!